Planck And Photoelectric Effect


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Planck And Photoelectric Effect

  1. 1. Light, Energy, and More October 23, 2007 Chemistry
  2. 2. Recap… <ul><li>Electromagnetic Spectrum </li></ul><ul><li>High Energy </li></ul><ul><li>Low Energy </li></ul><ul><li>Wave Nature of Light </li></ul>
  3. 4. What’s Going On Here?
  4. 5. When we heat metal what happens?
  5. 6. <ul><li>Does the wave model of light explain these changes? </li></ul><ul><ul><li>Does not explain different wavelengths and frequencies at different temperatures </li></ul></ul><ul><li>What is light? </li></ul><ul><ul><li>Radiation….what is radiation? </li></ul></ul><ul><ul><ul><li>Particles or rays of energy </li></ul></ul></ul><ul><li>What is temperature anyways? </li></ul><ul><ul><li>The measure of the average kinetic energy of the particles in an object </li></ul></ul><ul><ul><li>Kinetic Energy vs. Potential Energy </li></ul></ul><ul><li>Too many questions…. </li></ul>
  6. 7. Max Planck (1900) <ul><li>German Physicist </li></ul><ul><li>Began to look for answers </li></ul><ul><li>Matter can only gain or lose energy in small quantized amounts </li></ul><ul><li>What’s quantized? </li></ul>
  7. 8. Vocab Word!!! <ul><li>QUANTUM </li></ul><ul><ul><li>Minimum amount of energy that can be gained or lost by an atom </li></ul></ul><ul><ul><li>The emitted light from a glowing metal is a ENERGY…this energy is quantized </li></ul></ul>
  8. 9. If energy is now quantized…how can we determine the amount of energy of a quantum? <ul><li>What is energy measured in? </li></ul><ul><li>What are we observing? </li></ul><ul><li>What happens to the color when we increase the temperature (energy)? </li></ul><ul><ul><li>Proportional or inversely proportional? </li></ul></ul><ul><li>Now we need a constant… </li></ul><ul><ul><li>Planck’s constant, h=6.626 x 10^-34 J*s </li></ul></ul>
  9. 11. Time to put these words into action! <ul><li>What is the frequency and wavelength electromagnetic radiation that emits 1.68 x 10^-17 J of energy? What type of electromagnetic radiation is this? </li></ul><ul><li>Wavelength= 1.18 x 10^-8 m </li></ul><ul><li>Ultraviolet radiation </li></ul>
  10. 12. Some questions to answer… <ul><li>What is the color we see? </li></ul><ul><li>What happens to the energy of the radiation when we increase the frequency, v, of the radiation emitted? </li></ul><ul><li>Iron at room temp…color and E? </li></ul><ul><li>Iron with a little heat…color and E? </li></ul><ul><li>Iron with lots of heat…color and E? </li></ul>
  11. 13. According to Planck’s Theory… <ul><li>If we have a given v, matter can emit or absorb E only in whole number multiples of hv (1hv, 2hv, 3 hv…) </li></ul><ul><li>Matter can ONLY have specific amounts of energy </li></ul><ul><li>Wall of kids building blocks </li></ul><ul><ul><li>We can only add or take away in increments of whole blocks…we cannot remove half a block </li></ul></ul>
  12. 14. The Big Mystery of the 1900’s… <ul><li>The Photoelectric Effect… </li></ul><ul><ul><li>What caused these color changes in metals??? </li></ul></ul>
  13. 15. Photoelectric Effect <ul><li>Electrons (photoelectrons) are emitted from a metal’s surface when a light of a certain frequency shines on the surface </li></ul><ul><li>Certain specific amounts of energy (what’s this called???) needed to knock out electrons from metal atoms. </li></ul>
  14. 17. Albert Einstein (1905) <ul><li>Added onto Planck’s Theory… </li></ul><ul><li>Called the electron’s emitted, PHOTONS (the little energy packets Planck called quantums) </li></ul><ul><li>Now… E photon = hv </li></ul>
  15. 18. <ul><li>Planck paved the way for the explanation behind the mystery </li></ul><ul><li>But some one else came into the picture… </li></ul>
  16. 19. Now light is not just a wave… <ul><li>Einstein’s Dual Nature of Light </li></ul><ul><ul><li>Particle and wave characteristics </li></ul></ul><ul><ul><li>Light is a beam of tiny particles, called photons, acting like a wave </li></ul></ul>
  17. 20. NEW WORD!!! <ul><li>Photon </li></ul><ul><li>A particle of electromagnetic radiation with no mass that carries a quantum of energy </li></ul>
  18. 22. What Einstein added… <ul><li>Energy of a photon has a minimum or threshold value to eject photoelectrons </li></ul><ul><li>What must happen for the photoelectric effect to occur? </li></ul><ul><ul><li>Energy of a photon (particle of EM radiation) must have the minimum energy requirement to free the electron from the atom of metal </li></ul></ul>
  19. 23. Mystery Solved! <ul><li>No matter how long a light of a certain frequency is shone on metal (intensity), electrons will not be ejected unless the minimum amount of energy is shone. </li></ul><ul><li>Silver metal </li></ul><ul><ul><li>Photoelectrons ejected when a light with a frequency of at least 1.14 x 10^15 Hz or greater is used </li></ul></ul><ul><li>Sodium metal </li></ul><ul><ul><li>Red light </li></ul></ul><ul><ul><li>Violet light </li></ul></ul>
  20. 24. Revised Planck’s Work… <ul><li>Einstein piggy-backed off of Planck’s Theory and we now have….. </li></ul>Photon
  21. 25. Time to do a little work…. <ul><li>Tiny water drops in the air disperse the white light of the sun into a rainbow. What is the Energy oa a photon from the violet portion of the rainbow if it has a frequency of 7.23x10^14 Hz? </li></ul><ul><li>E=4.79 x 10^-19 J </li></ul><ul><ul><li>Energy in a photon of violet light </li></ul></ul>
  22. 26. A couple more…  <ul><li>A photon has an energy of 2.93 x 10^-25 J. What is its frequency? What type of electromagnetic radiation is the photon? </li></ul><ul><li>V=4.42 x 10^8 Hz </li></ul><ul><li>TV or FM waves </li></ul>
  23. 27. Practice makes perfect…  <ul><li>What is the energy of each photon in the following types of radiation? </li></ul><ul><ul><li>6.32 x 10^20 Hz </li></ul></ul><ul><ul><li>9.50 x 10^13 Hz </li></ul></ul><ul><ul><li>1.05 x 10^16 Hz </li></ul></ul><ul><li>What types of radiation are each? </li></ul><ul><li>4.19 x 10^-13 J gamma or x-ray </li></ul><ul><li>6.29 x 10^20 J infrared </li></ul><ul><li>6.96 x 10^-18 J ultraviolet </li></ul>